The mammalian immune system plays a vital role in protection from disease, but its effectiveness rests on the equilibrium between different responses. Excessive or inappropriate responses can result in autoimmune disease, while a failure to respond results in immunodeficiency. When such conditions occur, therapeutic intervention may be required.
The mature cells of the immune system, T cells, B cells and natural killer cells, continually differentiate from hematopoietic stem cells, through a series of cell divisions. It is believed that after each cell division the developmental potential of the daughter cells is either maintained or further restricted relative to the parent, never expanded. One therefore observes that pluripotential stem cells give rise to multi-lineage committed progenitor cells, which give rise to specific lineages and finally mature cells. The coordinated changes of cellular properties leading to irreversible restriction of lineage commitment may be due to sequential activation or silencing of various genes.
The phenotype of long-lived pluripotential hematopoietic stem cells has been described. However, the identification of intermediate bipotent or oligopotent progenitors has been difficult, since the evaluation of differentiating potential may be perturbed by a possible failure for the cells to read out detectable differentiation to particular lineages, which may be due to failure in reaching suitable microenvironments in vivo, an insufficient expansion for detection in vivo, or the stochastic nature of lineage commitment, at least in vitro.
The use of pluripotential or lineage committed progenitor cells circumvents many of the problems that would arise from the transfer of mature cells. However, such progenitor cells must be separated from other hematopoietic cells. Separation requires identification of the cell and characterization of phenotypic differences that can be utilized in a separation procedure. Cells that are amenable to genetic manipulation are particularly desirable.
Relevant Literature
A number of review articles have been published addressing the phenotype of cells in hematopoietic lineages. Overall development of the hematolymphoid system is discussed in Orkin (1996) Curr. Opin. Genet. Dev. 6:597-602. The role of transcriptional factors in the regulation of hematopoietic differentiation is discussed in Georgopoulos et al. (1997) Annu. Rev. Immunol. 15:155-176; and Singh (1996) Curr. Opin. Immunol. 8:160-165.
The phenotype of hematopoietic stem cells is discussed in Morrison & Weissman (1994) Immunity 1, 661-673; Spangrude et al. (1988) Science 241, 58-62; Enver et al. (1998) Blood 92, 348-351; discussion 352; Uchida et al. (1994) Blood 83, 3758-3779, Morrison et al. The aging of hematopoietic stem cells. Nat Med 2, 1011-1016 (1996).
The phenotype of a common lymphoid progenitor cell is discussed by Kondo et al. (1997) Cell 91, 661-672. The role of Bcl-2 in lymphopoiesis is discussed in Akashi et al. (1997) Cell 89, 1033-1041. Lineage commitment and maturation is discussed by Metcalf (1998) Blood 92, 345-347; discussion 352. Mice defective in two apoptosis pathways in the myeloid lineage develop acute myeloblastic leukemia; Traver et al. (1998) Immunity 9, 47-57 (1998). Multipotent progenitors in acute myelogenous leukemia are described by Miyamoto, et al. (1996) Blood 87, 4789-4796.
The transcription factor GATA-1 is described by Shivdasani (1997) Embo J 16, 3965-3973 (1997); Pevny et al. (1991) Nature 349, 257-260. Zon et al. (1991) P.N.A.S. 88, 10638-10641.